1. Field of the Invention
The present invention relates to an optical tunable filter and a method for manufacturing the optical tunable filter.
2. Description of the Prior Art
As for patents related to the optical tunable filter according to the present invention, the following documents can be mentioned.
<Filter Formed by Surface Micro-Machining>
In a conventional optical tunable filter, the thickness of a variable gap is controlled only by the thickness of a sacrifice layer. According to such a method, variations occur in the thickness of the variable gap depending on conditions for forming the sacrifice layer, thus resulting in a problem that a uniform Coulomb force is not generated between a thin film and a drive electrode so that stable driving cannot be achieved. Further, since a conventional optical tunable filter has a structure in which a movable portion protrudes from the surface of a substrate, the optical tunable filter is large in thickness (see Japanese Patent Laid-open No. 2002-174721, for example).
<Filter Using SOI Wafer>
On the other hand, U.S. Pat. No. 6,341,039 discloses a filter having a variable gap formed using an SiO2 layer of an SOI (Silicon on Insulator) wafer as a sacrifice layer. By using such an SiO2 layer of an SOI wafer as a sacrifice layer, it is possible to form a variable gap with high accuracy. In this filter, however, an insulating structure is not provided between a drive electrode and a movable portion, thus resulting in a problem that the movable portion and the drive electrode stick together when a large electrostatic attraction is generated therebetween (see U.S. Pat. No. 6,341,039, for example).
<Problem Common to Both Types of Filter>
In both types of filter, the sacrifice layer is ultimately released to form the variable gap. Therefore, a release hole is necessarily provided in the filter in order to feed a liquid for releasing to the sacrifice layer. This causes a problem that an area where Coulomb force acts is reduced so that a voltage for driving is increased. Further, if the variable gap is small, a phenomenon, in which the thin film and the drive electrode substrate stick together due to the surface tension of water, occurs when the sacrifice layer is released (that is, a phenomenon referred to as “sticking” occurs). Under the circumstances, there is a demand for a filter which can be manufactured without releasing a sacrifice layer.